Urban Dictionary: motorboat

v. int. The act of pushing one's face in between two ample breasts, and rocking one's head side to side very rapidly while making a vigorous, lip-vibrating "brrr" sound. n. The sound produced when doing a motorboat.

[kanan5100]

へえ。“To submerge ones face in large breasts and moving your head back and forth, flapping your lips, in turn making a motorboat sound.”